1. Field of the Invention
The invention relates generally to a technology pertaining to a laser welding apparatus and a laser welding method, and more specifically to a technology for enhancing the speed of laser welding.
2. Description of Related Art
Laser welding apparatuses and laser welding methods for joining two members together by laser welding are widely used. Examples of products that are manufactured by joining two members together by the laser welding apparatuses and the laser welding methods include secondary batteries including a box-shaped battery case and a lid member. Such secondary batteries are usually manufactured by joining a lid member to an open portion of a battery case by laser welding. One example of the technology for joining a lid member to an open portion of a battery case by laser welding is described in Japanese Patent Application Publication No. 2012-110905 (JP 2012-110905 A).
With a laser welding method and a laser welding apparatus described in JP 2012-110905 A, a case (battery case) and a sealing plate (lid member) are welded together by applying a laser beam to the battery case and the lid member. The laser beam is formed of a low power density laser beam and a high power density laser beam. With the conventional laser welding method and laser welding apparatus described in JP 2012-110905 A, the low power density laser beam is applied to the battery case, the lid member and a clearance between the battery case and the lid member in whole, while the high power density laser beam is applied to a portion of each of the battery case and the lid member, which is located within the area to which the low power density laser beam is applied. The laser welding method and laser welding apparatus described in JP 2012-110905 A make it possible to stably weld the battery case and the lid member together with high quality.
The low power density laser beam, which is used in the laser welding method and laser welding apparatus described in JP 2012-110905 A, is applied in order to avoid abrupt temperature changes due to the high power density laser beam. Thus, the intensity of the low power density laser beam is not high enough to melt and evaporate the material. For this reason, with the conventional laser welding method and laser welding apparatus, it takes a long time for molten material to flow into the clearance. Consequently, with the conventional laser welding method and laser welding apparatus, it takes a long time to fill the clearance with the molten material. Thus, an increase in the laser beam (high power density laser beam) scanning speed may result in entrance of the laser beam into the clearance. Note that, in the following description, a phenomenon, in which a laser beam enters a clearance between two members that constitute a workpiece subjected to welding and then the laser beam reaches the inside or back side of the workpiece, will be referred to as laser beam penetration.
In the case where the laser welding method and laser welding apparatus described in JP 2012-110905 A are employed, when the inside or back side of a workpiece subjected to welding is provided with a part that should be protected, it is necessary to set the welding speed to a speed at which laser beam penetration does not occur. This makes it difficult to enhance the speed of laser welding. In view of this, there has been a demand for development of a technology pertaining to a laser welding apparatus and a laser welding method, which makes it possible to enhance the speed of laser welding that is performed to join two members together while avoiding occurrence of laser beam penetration.